1. Field of the Invention
This invention relates to the continuous casting of thin carbon steel strip and, more particularly, to such casting of a liquid steel containing carbon in a critical maximum amount of about 60 parts per million (ppm) (0.006 weight percent) to produce a product of low strength and high ductility which later may be strengthened, as by carburizing or nitriding the cast strip.
2. Description of the Prior Art
Continuous casting of carbon steels in the form of slabs having a thickness in the range, e.g., of 8 to 10 inches, at high casting speeds, e.g., 30 to 80 inches per minute (ipm), has become very common in the steelmaking art, and today still is the conventional way to cast carbon steels. Such thick slab casting technology is well established for nearly all ranges of carbon level, including ultra-low (0.005% max.) carbon interstitial free steels, suitable for a wide variety of applications. Such technology includes the casting of very low carbon steels having relatively low strength and high ductility. An example is the use of such compositions in the manufacture of enameling steels, such as disclosed in Japanese patent numbers 60-110,845 and 60-221,520. To similar effect is U.S. Pat. No. 5,460,665 disclosing the manufacture of a conventionally cast, hot rolled, cold rolled and annealed sheet of steel having an ultra-low carbon content of 0.004% maximum. As disclosed in the latter patent, the manufacture of sheets or strip of such steels may involve post-casting processing, such as hot rolling, pickling, cold rolling, and recrystallization annealing.
Recently, there has been a trend, especially in the mini-mill sector, to cast thinner slabs (e.g. 2 to 4 inches thick) and at higher casting speeds, and the technology has been developed to produce steels with all ranges of carbon common to thick slab cast steels. This trend has further developed production of even thinner cast products. For example, Japanese patent number 61-133,324 shows the use of low carbon (up to 0.007%) steel in the production of thin steel ingots reduced by rolling to a thickness below 50 mm. Similarly, U.S. Pat. No. 4,586,966 discloses the production by continuous casting of thin (e.g. 10-40 mm) cast plate of low carbon (0.001-0.015%) steel which is directly cold rolled and annealed.
In the manufacture of the above-mentioned products, it is known to add certain carbide, nitride and sulfide formers, such as titanium, niobium, vanadium, zirconium, boron, etc. to affect the properties of the cast and processed steel, e.g. by forming strengthening particulates of such elements. For example, the low carbon, slab-cast enamelling steel of Japanese Patent No. 60-110,845, mentioned above, contains 0.05-0.12% titanium in order to improve the steel surface, enhance press formability and avoid fish scaling. The above-mentioned U.S. Pat. No. 4,586,966 adds titanium, niobium or zirconium to the 0.0010 to 0.015%C steel in order to remove nitrogen as nitrides of these additive elements. U.S. Pat. No. 5,578,143 is directed to the continuous slab casting of interstitial free (IF) steels of low carbon content (up to 0.005% in the base metal, and 0.01-0.08% in a surface layer) and with the addition of at least one of titanium, niobium or zirconium to combine with the carbon and nitrogen as carbides, nitrides, or carbonitrides, of the respective additives.
It is also known in the art to strengthen conventionally cast low carbon steels by carburizing or nitriding them, generally to form a hard outer layer or case on the steel. These processes may proceed by known means such as liquid or, more commonly, gas carburizing, e.g. in a natural gas atmosphere, or by nitriding, e.g. in an ammonia-containing gas atmosphere as described in U.S. Pat. No. 3,928,087, or U.S. patent application Ser. No. 08/773,205, filed Dec. 23, 1996 and assigned to the assignee hereof, which application is incorporated herein and made a part hereof by this reference.
A third technique of continuous casting of carbon steels is currently being developed; that is, strip casting at low product thicknesses, e.g. about 0.1 inch or less, and at very high casting speeds, e.g. about 1000-6000 inches per minute (ipm). Examples of thin strip casting include U.S. Pat. No. 5,484,009 disclosing a casting method and apparatus wherein liquid steel is partially cooled by a rotating casting roll, leaving an upper surface of the cast strip in liquid form which subsequently is solidified. U.S. Pat. No. 5,520,243 discloses metal strip casting wherein quality of the cast strip is a function of the roughness of the casting and cooling roll, and the metal is vibrated during casting, providing possible thicker strip with higher K value.
Metallurgically, strip casting of carbon steels is very different from conventional thick slab casting or even thin slab or plate casting, in that the cooling rates to which the strip cast steel is subjected are much higher, e.g. on the order of 2000.degree. C. per second, and rates as high as 10,000.degree. C./second may be involved. Such extremely high cooling rates are required in strip casting to be sure that the strip, or at least a substantial part of the thickness thereof, is solidified before leaving the mold or cooling roll surface at the extremely high casting speed necessary for practical commercial production justifying the capital investment and maintaining a competitive operating cost. The metallurgical structure produced in carbon steels is very dependent on the cooling rate during casting. Too high a cooling rate will produce undesirable phases such as acicular ferrite, bainite, or martensite, as exemplified in FIG. 1 below. These phases are much higher in strength and lower in ductility than the typical ferrite structure produced with lower cooling rates for conventional thick slab or thin slab casting. These latter cooling rates are sufficiently low that these undesirable phases are not present in sufficient quantity to adversely affect the strength or ductility of the cast products. On the other hand, the high casting speeds and resulting required high quenching rates inherently associated with thin strip casting produce a cast strip with the undesirable properties, such as high hardness and brittleness, resulting from such unavoidable metallurgical structure. Coiling of such hard, brittle strip may result in strip cracking problems. It has been suggested that "the unique metallurgical structure of acicular ferrite, bainite and martensite found in thin strip cast products is a challenging starting point for subsequent thermomechanical processing of such cast strip in order to convert the cast microstructure to an acceptable condition having better mechanical properties". (AISI Strip Casting Update: July 1997) Such additional, post-casting processing might include high temperature anneals, e.g. austenitization followed by slow cooling--which could cause scaling problems--and then pickling. Thus even if the postulated thermomechanical processing of thin cast steel strip successfully changes the undesirable cast phases to acceptable ones, the achievement likely will be at the price of further processing yield losses and costs.